SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Aug. 14, 2019 USD ($) $ / shares shares	Jun. 30, 2016 shares	Dec. 31, 2020	Dec. 31, 2020 USD ($)	Dec. 31, 2020 company	Dec. 31, 2020 item	Dec. 31, 2020 state	Dec. 31, 2019 USD ($)	Aug. 13, 2019
Number of passive investment subsidiaries					1	1
Shares issued | shares	31,036,812
Shares issued (in dollars per share) | $ / shares	$ 10.00
Net proceeds from sale of common stock	$ 310,400							$ 304,061
Expenses incurred	$ 6,300							6,300
Stock conversion ratio	1.795431
Shares issued in exchange | shares	12,162,763
Net proceeds from sale of common stock	$ 304,100
Amount utilized to fund new ESOP	24,800							$ 24,829
Number of full-service bank offices | item						25
Number of limited-service bank offices | item						1
Impairment of federal home loan bank stock				$ 0
Number of reporting units | item						2
Number of security corporations | item						2
Impairment recognized of goodwill and intangible assets				$ 0
LTV 80 to 100 Percent | Residential
Loan To Value Ratio			80.00%
HarborOne Mortgage
Number of offices | item						36
Additional states licensed to lend | state							4
MHC | HarborOne Bancorp Inc.
Ownership percentage									53.00%
HarborOne Foundation
Shares issued | shares		385,450
HarborOne Bank
Number of security corporation subsidiaries | company					2
Cash invested into Bank's operations	$ 151,300